You pretty much have it all figured out.... to go point by point...

1. correct
2. You'll need a brass fitting with a barb for your vacuum hose... parts shop or hardware store will have these.
3. Pull the drivers' side kick panel and find the boost gauge line under there. The stock vac line in my car already had a break with a coupler maybe a foot from the back of the cluster... perfect spot for the T.
4. I used 5/32" hose... pretty close to the 4.2 mm Volvo vac hose size. Hose length will depend on where you put the gauge. Go nuts and buy a bunch... it's only 70 cents a foot or so, and you'll use the rest under the hood at some point.
5. You just need the barb fitting for the gauge, vac hose, and a T. Plus a flashlight to see what you're doing under the kick panel. Wiring up the bulb is pretty easy, too.
6. That's it, go for it.

Where are you going to mount your gauge? For me, figuring that out (left side of steering clolumn) was a lot harder than actually plugging in the lines, etc.

Have fun... you'll enjoy the g-valve.

Brian '93 945T